This invention relates in general to photodetection systems.
Photodetection systems have been widely used in a number of different applications. For example, such systems have been used in weighing scales. Photodetection systems have also been used as optical encoders in conjunction with motors for determining the position of a rotating member during rotation.
One type of convention photodetection system used for the above-described application, is described in U.S. Pat. No. 4,654,525. According to this patent, an optical rotary encoder includes a circular slit plate having a number of slits located at the circumference of the plate, where the slits have a pitch P. A photodiode emits light towards one side of the slit plate and four photodiodes are placed on the other side of the slit plate to detect the light emitted by the light emitting diode through the slits. Output signals from the four photodiodes are applied to a detection circuit for determining the angular position and velocity of the rotating slit plate. In this manner, the angular position and velocity of a rotating shaft used to rotate the slit plate can be measured.
The amount of light detected by the photodiodes is proportional to the surface area of the photodiodes receiving light from the light emitting diode. Thus, in order to increase the strength of the signal detected by the photodiodes, it is preferable to employ photodiodes of large areas, or to employ multiple sets of smaller photodetectors. This, however, will increase the size of the optical head containing the photodiodes, which may be impractical for many applications. This is particularly the case for the increasingly popular portable electronic devices and in view of the modem trend to miniaturize electronic devices. Furthermore, a larger area photodetector causes the dark current to increase, thereby reducing the signal-to-noise ratio, and causes capacitance to increase, thereby reducing speed of devices.
Another technique to increase the intensity of the detected signal is to amplify the output of the photodetector, such as in the manner shown in FIG. 6 of U.S. Pat. No. 4,654,525. As shown in FIG. 6 of such patent, an operational amplifier with negative feedback is employed to amplify the output of each photodiode. The use of feedback, however, renders the detection circuit less stable. It is, therefore, desirable to provide an improved photodetection system where the above-described disadvantages are avoided or alleviated.